Basic elements of a conventional steam power plant 10 are illustrated in schematic form in FIG. 1. A boiler 12 burns a combustible fuel to provide heat energy to convert feedwater into saturated or superheated steam for delivery to a high-pressure turbine 14. The steam is expanded through the turbine 14 to turn a shaft that powers an electrical generator (not shown). The steam is then directed in sequence through an intermediate pressure turbine 16 and a low-pressure turbine 18 where additional shaft energy is extracted. The spent steam leaving the low-pressure turbine 18 is converted back to water in condenser 20. A condensate pump 22 delivers water from the condenser 20 to a low-pressure feedwater heater 24. The feedwater heater 24 is a heat exchanger that adds energy to the water as a result of a temperature difference between the water and steam supplied through a low-pressure steam extraction line 26 from the low-pressure turbine 18. The heated water is collected in a feedwater tank 28 which is also provided with an intermediate-pressure steam extraction connection 29. From the feedwater tank 28, the water is delivered by a feedwater pump 30 through an intermediate pressure feedwater heater 32 and high-pressure feedwater heater 34, where additional energy is supplied via the temperature difference between the water and steam supplied through intermediate pressure steam extraction line 36 and high-pressure steam extraction line 38 respectively. The heated feedwater is then delivered back to the boiler 12 where the cycle is repeated. Plant 10 may include many other components, systems and subsystems that are not illustrated in FIG. 1 but that are well known in the art. Other known steam power plant designs may utilize fewer or additional pressure stages for both energy extraction and feedwater heating.
The power plant 10 of FIG. 1 is a heat engine with a vapor cycle commonly referred to as a Rankine cycle. An ideal Rankine cycle consists of four processes: isentropic expansion through an expansion engine such as a turbine, piston, etc.; isobaric heat rejection through a condenser; isentropic compression through a pump; and isobaric heat supply through a boiler. FIG. 2 is a typical Ts diagram illustrating the relationship of entropy and temperature for a prior art Rankine cycle 39 such as may be implemented in prior art power plant 10. The dashed line represents the vapor dome underneath which the working fluid (water for most commercial power plants) will exist in both the liquid and vapor states simultaneously. Saturated or superheated steam enters a turbine at state 40, where it expands to the exit pressure at state 42. This expansion is not completely isentropic due to the expected inefficiencies in the turbine design. The steam is condensed at constant pressure and temperature to a saturated liquid at state 44. The saturated liquid then flows through condensate pump that increases the pressure to state 46. The pressurized water is heated through the low-pressure feedwater heater 24 to state 48 and further pressurized to boiler pressure by feedwater pump 30 to state 50. The water is then further heated through intermediate pressure feedwater heater 32 and high-pressure feedwater heater 34 to states 52, 54 respectively. The water is then heated to saturation temperature, boiled and typically superheated back to state 40 in boiler 12.
The rising cost of fuel and the demand for lower emissions provide a continuing need for improvements in the efficiency of operation of steam power plants.